Galactic cosmic rays measured by UVS on Voyager 1 and the end of the modulation

Abstract

The detectors of the Ultraviolet Spectrographs (UVS) on Voyager 1/2 are recording a background intensity that was earlier assigned mainly to disintegrations in the radio-isotope thermoelectric generator and systematically subtracted from the signal to infer photon counting. Here, we show that it arises instead from galactic cosmic rays (GCRs). We show the GCR flux measured by UVS on Voyager 1 from 1992 to August 2013, and by comparing to data from the GCR dedicated detectors, we estimate the energy range responsible for this UVS signal, which is around 300 MeV, and the response of UVS to the GCR anisotropy. After the abrupt jumps of May and August 2012, the count rate has been only slightly fluctuating around a constant value. However, comparing it to the data from the Low Energy Charge Particle Experiment (LECP) and the Cosmic Ray Subsystem (CRS) shows that those small variations are only responses to a varying anisotropy and not to a flux change. Taking advantage of the similarity in energy range to one of the products of the CRS instrument suite, we use the ratio between the two independent signals as a proxy for the temporal evolution of the GCR spectral slope around the 300 MeV range. We show that this slope remained unchanged since August 2012 and find strong evidence that it will no longer vary, implying the end of the heliospheric modulation at those energies, and that Voyager 1 at this date is near or past the heliopause. The origin of this unexpectedly narrow and stagnating inner heliosheath is still unclear, and we discuss the potential effects of low solar wind speed episodes and subsequent self-amplified charge-exchange with interstellar neutrals, as a source of deceleration and collapse. We suggest that the quasi-static region encountered by Voyager 1 may be related to such effects, triggered by the strong solar-maximum variability. This did not happen for Voyager 2 due to its trajectory at an angle further from the heliosphere axis and a later termination shock crossing. The existence on the upwind side of a mixing layer formed by charge transfer instead of a pure plasma contact discontinuity could explain various Voyager 1 observations.

Voyager 1/UVS Lyman α Measurements at the Distant Heliosphere (90–130 AU): Unknown Source of Additional Emission

“…The particle background spectrum was measured during dedicated observations and was found to be very stable over the years, with very little channel to channel variation (see supporting information in Lallement et al, ). Lallement et al () have studied in detail the temporal evolution of the Voyager 1 UVS background since 1993. They found that there is a perfect correlation between the UVS background and the Cosmic Ray Subsystem (CRS) signal from energetic particles with energy above 70 MeV (see Figures 1 and 2 in Lallement et al, and Figure a for an illustration).…”

Probing the Local Bubble with diffuse interstellar bands

“…While the putative million-degree gas was not seen in the expected extreme-UV emission by the CHIPS satellite (Hurwitz et al 2005) nor in O vi 1032 Å absorption (Cox 2005;Barstow et al 2010), recent works making use of soft X-ray shadows, Planck mm-wavelength data, and Local Bubble maps have now considerably reinforced the idea that there is such hot gas within the cavity (Snowden et al 2015). The picture is complicated by the interaction between the heliosphere and surrounding ISM, such as charge exchange between the solar wind ions and interstellar atoms (e.g., Lallement et al 2014b;Zirnstein et al 2015; but see Galeazzi et al 2014), indicating local "fluff" of a more neutral kind (Frisch 1998). Any novel way or tracer that can be used to map the Local Bubble is therefore highly desirable.…”

Magnetic Flux Conservation in the Heliosheath Including Solar Cycle Variations of Magnetic Field Intensity

“…These time variable magnetic barriers cause the radial velocity to decrease within this region (Pogorelov et al 2009). Finally, Lallement et al (2014) suggest that, as the flow speed decreases to 10 km s −1 , the charge-exchange rate increases considerably, leading rapid momentum loss and further deceleration in the HS.…”

“…The non-conservation of magnetic flux at V1 could be explained if reconnection were taking place within the sector region (Drake et al 2010;Opher et al 2011) since V1 was immersed within this region throughout the HS while V2 went in and out. Lallement et al (2014) also note that in the stagnation region, where a large fraction of the plasma is undergoing charge-exchange, magnetic flux may not be conserved as well.…”